Ions - Sodium and Potassium - Online Tutor, Practice Problems & Exam Prep

The movement of sodium and potassium ions is driven by the electrochemical gradient, which combines electrical and concentration gradients. Sodium is typically more concentrated outside resting neurons, while potassium is more concentrated inside. The sodium-potassium pump actively transports 3 sodium ions out and 2 potassium ions into the cell, counteracting their natural diffusion tendencies. This process is crucial for maintaining resting membrane potential and overall cellular homeostasis, highlighting the importance of active transport in cellular function.

Electrochemical Gradient

So in this video, we're going to be talking about sodium and potassium ions, more specifically, the force that moves them, the electrochemical gradient. Now, this is basically a combination of two factors, the electrical gradient and the concentration gradient. So, the electrical gradient states a principle that I bet you've heard of before, opposites attract. So, the electrical gradient makes ions move toward an area of opposite charge. Positively charged ions are attracted to negatively charged spaces and vice versa.

Now, the concentration gradient states that ions move from areas of high concentrations to areas of low concentrations. So you can think of ions as kind of like introverts. They want to get out of a crowd and move toward an area of low concentration, which, like, same honestly. One thing to know about the concentration gradient is that the greater the difference in concentrations of ions between the inside and outside of the cell, the more rapid the diffusion or movement of those ions is going to be. A greater difference in concentration gets you more rapid diffusion or more rapid movement of our ions.

This is sometimes called the chemical gradient, so if you see that people are talking about the concentration gradient. I prefer calling it the concentration gradient because to me it's more intuitive because it's stating that ions move from areas of high concentrations to low concentrations, but you can call it either one. They're both correct. Now, these two gradients together make up the electrochemical gradient. These two gradients can be moving ions in the same direction. For example, they could both be moving sodium ions out of a cell, but sometimes they can be opposing. One gradient could be moving sodium out of a cell and one could be moving sodium into the cell, for example. And if that happens, if they're opposing, whichever gradient is strongest is going to be driving that net flow of ions.

So let's just dive right into an example. So here, we're going to be drawing an arrow in each of these boxes to indicate which direction the electrical and chemical or concentration gradients would be directing the flow of our potassium ions. Just to orient you to this image here, we are sitting pretty on the membrane of this lovely neuron and this orange area here is our cell membrane. This tan area is our cytosol or the inside of our cell and this blue area here is our extracellular fluid. A few things I'm noticing right away is that our extracellular fluid is positive and our cytosol is negative and I see that we're working with the potassium leak channel. We know all about leak channels, right? These channels are always open and ions can move freely in and out of them, and I see that we have more potassium ions inside the cell.

I'm counting 6 ions inside the cell and 4 ions outside the cell, so our area of higher concentration is in our cell. So knowing all that information, let's figure out those gradients. Potassium is a positively charged ion and opposites attract, right? So it's attracted to this negatively charged cytosol and so our electrical gradient would be directing ions into the cell. Now remember for the concentration gradient, ions are kind of like introverts. Right? They want to get out of a crowd and go from high to low concentrations. So in our case, our area of high concentration is inside the cell and our area of low concentration is outside the cell. And so our concentration gradient is moving ions out of the cell. This is a very simplified example and even though these gradients are opposing, we wouldn't have enough information to know which one is strong enough to drive the net flow of ions, but hopefully, you kind of get the idea.

So, I will see you guys in our next video where we're going to talk even more about sodium and potassium. So, I'll see you there.

Suppose the extracellular fluid has a chloride ($Cl^-$) concentration of 120mM, while the concentration of chloride ($Cl^-$) inside the cytosol is 60mM. Also suppose that the total net charge of the cytosol is more negative than the extracellular fluid. Given this information, which statement is correct regarding the movement of $Cl^-$ ions?

• A. The electrical and concentration gradients both favor movement into the cell.
• B. The concentration gradient favors a net movement out of the cell, the electrical gradient favors movement into the cell.
• C. The electrical and concentration gradients both favor movement out of the cell.
• D. The concentration gradient favors a net movement into the cell, the electrical gradient favors movement out of the cell.

Standard Sodium and Potassium Concentrations

Okay. So now that we understand how and why those ions move, let's take a look at where those ions actually are. And for context, we'll be looking at these ion concentrations in a cell that is at rest. So our neuron is not communicating, just taking a little break. So, when our cell is at rest, what we typically see is that for sodium, we have a relatively low intracellular concentration, so not very much sodium inside of our cell, but we do have a very high concentration of sodium outside the cell; and we see the opposite pattern for potassium.

So for potassium in a cell that is resting, we tend to have a very high concentration inside the cell and a low concentration outside the cell. Alright, so easy peasy lesson there, right? Let's dive right into our example to kind of visualize this and practice those gradients a bit. So what we're going to be doing here is, given what we just learned about those sodium and potassium concentrations, we'll be drawing an arrow in these boxes to indicate which direction the concentration and electrical gradients will be directing the flow of these ions. So to quickly orient you to our image here, in case you haven't seen this before, the orange here is our membrane.

This tan area is our negatively charged cytosol. This blue area is our positively charged extracellular fluid. We have our pink potassium ion and our little purple sodium ion and our little pink potassium leak channel there, and our sodium leak channel. Remember, these guys are always open. Ions are coming in and out, and just looking at these pink and purple sodium and potassium ions, I can see that these concentrations are what we just talked about.

So I'm counting 1, 2, 3, 4, 5 potassium ions in the cell, our high concentration, and 2 outside the cell, that's our low concentration, and the opposite pattern for sodium. So I see 1, 2, 3, 4, 5 sodium ions out of the cell. That's our high concentration and just 2 inside the cell. That's our low concentration. So let's start with those concentration gradients since we've been talking about that this whole time.

So remember, ions are like introverts. Right? They want to get out of a crowd, go from their high to low concentrations. So for potassium, that's going to be directing the flow out of the cell. Right? The high concentration is in. They want to get away from that, so they're going to be going out of the cell. And for sodium, we'll see the opposite pattern. So our high concentration is outside the cell, so they're going to want to move into the cell. And so the electrical gradients for these are actually going to be the exact same because they're both positively charged ions, and remember, opposites attract, right, so they're both attracted to this negatively charged cytosol.

And so for both of these ions, the electrical gradients will be directing the flow into the cell. Alright. And you can see kind of a nice example here of what I mentioned in our last video how sometimes these gradients can be opposing, and sometimes they can be more synergistic. So for our potassium, we have these opposing gradients with the concentration gradient directing the flow out and the electrical gradient directing the flow in. And this is a cartoon.

Obviously, we don't have enough information to figure out which one is stronger and would drive net flow, but it gives you kind of a nice idea and a nice comparison to sodium here where both gradients are directing the flow into the cell. So you can tell we're going to have a pretty strong net flow of sodium. Alright. So that is the sodium and potassium concentrations of a cell that is at rest, and I will see you guys in our next video.

In a neuron at rest, the concentration of __________ is higher outside the cell than in the cell, whereas the concentration of _________ is greater inside the cell than outside.

• A. Sodium; potassium.
• B. Potassium; sodium.

Which of the following statements is true regarding how the concentration gradient affects sodium ions when a cell is at rest?

• A. When a cell is at rest the concentration gradient has no effect on sodium ions.
• B. The concentration gradient drives sodium ions into the cell.
• C. The concentration gradient drives sodium ions out the cell.
• D. In a cell at rest the electrical gradient moves into the cell.

The Sodium Potassium Pump

Okay. So up until this point, we've talked about how ions can move passively via their electrochemical gradients, but you guys may remember that's not the only way that ions can move. Like we talked about way earlier in the course, active transport can move ions against their electrochemical gradients and they do this via ATP consuming pumps. Now, arguably, the most important ATP consuming pump in the human body and my personal favorite is the sodium potassium pump or the sodium potassium ATPase. Now this pump works by ejecting 3 sodium ions from the cell.

So they get kicked out and it transports 2 potassium ions back into the cell. So it's basically moving those ions against those natural gradients. Remember, when our cell is at rest our leak channels are open, sodium wants to leak in and potassium wants to leak out and our pump says uh-uh uh-uh and it kicks that sodium back out and it pulls that potassium back in. So if we're looking at our figure here, we have our lovely blue pumps and you can see here 3 ions are being ejected from our cell, so that's got to be our sodium. I'm going to label each of these as sodium.

Alright, so they're getting tossed out and I see 2 ions here getting pulled back into this cytosol. That's got to be our potassium, so I'll label that as potassium. Now it is very important for you to understand how this pump works and what this pump is doing. So to help you do that, I have some fun memory tools. Now this one I can't take credit for.

I first saw this in Jason's general bio class, and it was so stinking cute I had to take it. So the idea here is to imagine our neuron as a club. This is club intracellular. Club intracellular is located in the salty, salty sea of the extracellular fluid full of sodium, right? So we're in club intracellular.

There's music, there's lights, we're dancing, we're being responsible though, and we have our bouncer. The bouncer is the sodium potassium pump because this guy works a lot like a bouncer. He decides who gets in and who gets out, right? So what our bouncer is going to do is he's guarding the door and these 3 sodium ions are going to come and they're going to be like, hey, can we come in the club? And he's going to tell them because sodium is Na.

Right? So cute. And then these 2 potassium ions are going to come and be like, hey, can we come in the club? And he's going to tell them k because potassium's k. So cute.

Full credit to Jason. Love that. So that's a super fun way to imagine it. If you prefer less elaborate study methods, though, my favorite easy-peasy way to remember this is to just write out sodium as Na+ and potassium as K+ and then just count the characters. So Na+ has 3 characters and K+ has 2 characters.

That helps you remember we're working with 3 sodium and 2 potassium. My other favorite easy way to think about this is to remember a pumpkin. So just draw yourself a little pumpkin or something because remember we pump the K (potassium) into the cell and whatever we're doing to potassium the opposite is being done to sodium, so kind of an easy way to help you remember that. Now I'm giving you all these tools because, like I said, it's important to know how this pump is working. It's going to come up a few more times, and so be sure to take your time with this, study it, and I'll see you guys in our next video to talk more about the function of our sodium potassium pump.

See you there.

Ions - Sodium and Potassium Example 3

Alright. So let's run through our example. Our example reads, Terry was injected with a poison that blocks the sodium-potassium pump. With the sodium-potassium pump blocked, what will happen to the concentration of potassium inside the cell? All right.

So let's just kind of draw this out so that we remember what's going on. In a cell, we have higher concentrations of potassium inside and higher concentrations of sodium on the outside, right, which means that potassium, following its concentration gradient, wants to leak out of the cell and sodium wants to come into the cell. And our pump is working to kick that sodium back out, right, and bring that potassium back in. Okay? So that's what's going on.

And now that that's kind of fresh in our heads, let's think through these answer choices. So a reads that if the pump was blocked, the concentration of potassium inside the cell will increase. And that can't be right. Because if we just have potassium leaking out and there's nothing replacing it, it's definitely not going to increase. So a is out.

B reads the concentration of potassium will decrease, and that is correct. So if potassium is just leaking out and there's nothing replacing it, it's going to decrease over time. So let's just really quickly go through c to make sure that we understand why it's not correct. So c reads that the concentration of potassium will be unaffected, and that's just not true because, like we just talked about, potassium wants to be leaking out.

And now there's no pump replacing it. And so the concentration is certainly being affected in that it is decreasing. Right? So c is incorrect as well. Right?

So our answer is b, the concentration of potassium will decrease. And there you go.

Which of the following statements about the Sodium Potassium pump is correct? 

• A. The sodium potassium pump operates as a mechanically gated channel.
• B. The sodium potassium pump transports 3 potassium ions and ejects 3 sodium ions.
• C. The sodium potassium pump always helps ions move down their natural electrochemical gradient.
• D. The sodium potassium pump transports 2 potassium ions and ejects 3 sodium ions.

Here’s what students ask on this topic:

What is the electrochemical gradient and how does it affect ion movement?

The electrochemical gradient is a combination of two forces: the electrical gradient and the concentration gradient. The electrical gradient is based on the principle that opposite charges attract, so positively charged ions move toward negatively charged areas and vice versa. The concentration gradient, on the other hand, drives ions from areas of high concentration to areas of low concentration. Together, these gradients determine the direction and rate of ion movement across cell membranes. For example, sodium ions (Na⁺) typically move into cells where their concentration is lower, while potassium ions (K⁺) move out of cells where their concentration is higher.

How do sodium and potassium concentrations differ inside and outside a resting neuron?

In a resting neuron, sodium (Na⁺) and potassium (K⁺) concentrations are distributed unevenly. Sodium ions have a high concentration outside the cell and a low concentration inside. Conversely, potassium ions have a high concentration inside the cell and a low concentration outside. This distribution is crucial for maintaining the resting membrane potential and is actively managed by the sodium-potassium pump, which moves 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell against their concentration gradients.

What is the role of the sodium-potassium pump in maintaining cellular homeostasis?

The sodium-potassium pump (Na⁺/K⁺ ATPase) is essential for maintaining cellular homeostasis by actively transporting ions against their concentration gradients. It ejects 3 sodium ions (Na⁺) from the cell and imports 2 potassium ions (K⁺) into the cell. This process consumes ATP and is vital for maintaining the resting membrane potential, regulating cell volume, and ensuring proper nerve and muscle function. By counteracting the natural diffusion of Na⁺ and K⁺ through leak channels, the pump helps stabilize the internal environment of the cell.

How do electrical and concentration gradients influence the movement of potassium ions?

Potassium ions (K⁺) are influenced by both electrical and concentration gradients. The electrical gradient attracts K⁺ ions into the negatively charged cytosol of the cell. Meanwhile, the concentration gradient drives K⁺ ions out of the cell, where their concentration is lower. These opposing forces create a dynamic balance, and the net movement of K⁺ ions depends on the relative strength of each gradient. In a resting neuron, the concentration gradient typically has a stronger influence, causing a net efflux of K⁺ ions.

What happens to sodium and potassium ions during active transport?

During active transport, sodium (Na⁺) and potassium (K⁺) ions are moved against their electrochemical gradients by the sodium-potassium pump (Na⁺/K⁺ ATPase). This pump uses energy from ATP to transport 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. This process is crucial for maintaining the resting membrane potential and overall cellular homeostasis. By actively moving these ions, the pump counteracts their natural diffusion tendencies, ensuring that the cell's internal environment remains stable and functional.